Limited-angle, electromechanical rotary actuators have been in existence for decades. They are used in a variety of industrial and consumer applications, but they are particularly useful in the field of optical scanning, where an optical element is attached to an actuator output shaft, and is then rotated back and forth in an oscillating manner.
For example and as illustrated with reference to FIG. 1, it is common to attach a mirror to the output shaft of a rotary actuator in order to create an optical scanning system. In this application, the actuator/mirror combination can redirect a beam of light through a range of angles, or redirect the field of view of a camera so that it can observe a variety of targets.
Typical electromechanical rotary actuators used in the field of optical scanning are generally made from some combination of magnet, steel and coils of insulated “magnet” wire. These elements have been arranged in a variety of ways, but for the past twenty years, the most popular arrangement has been to use a simple two-pole rotor magnet, and a “toothless” stator design, similar to a slotless/brushless DC or AC synchronous motor, but having a simpler, single-phase coil arrangement.
The rotor within these actuators is typically made of a cylindrical magnet, onto which one or two shafts are attached in one way or another. Several known rotor assemblies are illustrated by way of example with reference to FIGS. 2, 3 and 4.
When this type of actuator is used for optical scanning, one shaft may be attached to a mirror and another shaft operable with a position sensor. The rotor assembly is typically supported on one side or both sides by ball bearings.
It will be helpful to review known actuator technology and make reference to known actuators to have the reader better understand the needs satisfied by embodiments of the present invention.
FIG. 5 illustrates a sectional view of a rotor magnet, stator and coil arrangement found in a typical conventional optical scanner of current state of the art. The stator is essentially tubular and made from a solid piece of magnetically conductive material such as cold rolled steel. For the rotor magnet having a diameter of 0.120 inches, a typical stator tube may have an outside diameter of 0.5 inches (around 12.7 millimeters), and an inside diameter of 0.196 inches (approximately 5 millimeters). The coil is made of turns of magnet wire, bonded to the inside wall of the stator steel tube using epoxy. Each side of the coil is formed as an arc, often occupying an approximately 90-degree arc on each side of the stator as herein illustrated. There is typically around a 0.007 inch gap between the outside wall of the rotor magnet and the inside wall of the coil, thus allowing the magnet to rotate freely. With continued reference to FIG. 5, the coil areas are designated “Coil plus” and “Coil minus” to indicate turns going into the page and turns coming out of the page, respectively.
FIG. 6 illustrates magnetic field lines found in a conventional optical scanner of the current state of the art as illustrated in FIG. 5, using a solid cylindrical diametral-magnetized rotor magnet. It can be seen that the magnetic flux lines must extend (“jump”) across a relatively large gap to reach the stator steel. The coil resides in between the magnet and the stator steel. When the coil is energized, a Lorentz Force is imposed on both the coil and the magnet. Since the coil is typically bonded to the stator and thus, held stationary, all of the force is conveyed to the rotor magnet. Since force is created on opposite sides of the magnet, the force being in the form of torque, the actuator creates torque and thus creates motion.
FIG. 9a illustrates one cylindrical rotor magnet and coil windings. As shown, the magnet essentially resides “inside” the coil. Steel which resides outside the coil is not shown in this illustration. The coil includes multiple turns of magnet wire. The long, straight portion of the coil is known as the “active portion” because this is the portion which contributes to torque on the magnet. The rounded portion of the coil is known as “end-turns”. The end turns do not contribute to torque production. They are merely there to connect the active portion on one side of the coil to the other side of the coil. However, any heating of the drive coil that results from current passing through it, also exists in the end turns. Thus, while the end turns do not contribute to torque production, they do contribute to heat, ohmic resistance and electrical inductance, all attributes which are detrimental to overall actuator performance. Therefore, there is motivation to keep the end-turns as short as possible in order to minimize these detrimental effects.
By way of further example with reference again to FIG. 9a, the coil is shown having its coil windings completely surrounding the magnet on top, bottom, left and right portions. This coil arrangement is typically not used in known actuators because the end turns as diagrammatically illustrated in this FIG. 9a would prevent a shaft from reaching the magnet. Instead, the end turns must be bent out of the way (or rather “formed”), as illustrated with reference to FIG. 9b. When the end-turns are formed in this way, this typically allows for the shaft (which is attached to the magnet) to “pass through” the end turns and result in what is effectively a “hole” formed in the coil. Of course, this means that the “end-turns” must be made undesirably longer in order to create such a “hole”. As will be illustrated later in the teachings of the present invention, such an undesirable feature is eliminated in actuators herein presented by way of example.
Such a conventional actuator arrangement provides some desirable benefits. One benefit is the relatively low coil inductance that results from the fact that the coil does not completely surround a closed steel core. Quite the contrary, the entire inside of the actuator is open, containing only the rotor magnet whose permeability is almost the same as that of air. Another benefit is that the rotor generally has no “preferred position”, meaning that once the rotor is positioned, power can be removed from the coil and the rotor will remain in that position. For optical scanning applications, the performance of this type of actuator is well suited for applications including laser marking and some laser graphic projection.
However, although this conventional actuator structure has been used successfully for optical scanning for more than two decades, the costs involved in forming the coil and then bonding the coil to the stator have prevented this type of actuator from being highly successful in certain consumer-grade applications, including point-of-purchase displays, 3D printers, and certain self-driving and assisted-driving automobiles, where low cost is paramount.
For the type of actuator whose arrangement is shown in FIG. 5 and end turns formed as above presented in FIG. 9b, the coil is the most difficult and thus most costly part to manufacture, because ideally, it must be wound in three dimensions. Coils of this type are generally shown in FIG. 2a of U.S. Pat. No. 4,090,112 (item 50); FIG. 1 of U.S. Pat. No. 5,313,127 (item 30); FIG. 8 of U.S. Pat. No. 5,424,632 (item 75); and FIG. 4 of U.S. Pat. No. 6,633,101 (item 34 and 42). Although some of the figures show all of the individual coil turns neatly formed and having very good copper packing, such coil windings are typically known not to be this neat. Because of the 3D nature of the coil winding, the individual turns often effectively compete for space, with turns “crossing over” each other, thus leading to sub-optimal current density distribution as well as sub-optimal heat sharing among the turns of the coil.
Nevertheless, once the coil is formed, inserting it into the stator is the next challenge. Because of the close proximity of the stator wall to the coil windings, the insulation on the coil can be scratched during the insertion process, leading to an instant, or latent “coil-to-case short” type of electrical failure.
Bonding the coil to the stator walls is another difficult manufacturing step for this type of actuator. Thermally-conductive epoxy is often used to bond the coil to the inside of the stator walls, but very often, air bubbles are formed in the bond, leading to sub-optimal heat removal. The required epoxy curing time presents another challenge.
Absent some external angle-limiting element, it is known that these typical actuators can spin freely within the stator, and take on any rotational position. However, this is undesirable for optical scanning applications because these applications only exercise a mirror over a relatively limited range of angles—generally no greater than 40 degrees mechanical peak-to-peak. Moreover, when a single coil is used along with a two-pole magnet, a desirable torque is not produced at all rotational angles, and in fact no torque at all is produced at certain angles. For these reasons, an external rotational limit is imposed on this type of actuator. Most often, this limit is imposed by a “stopping pin”, which is driven through one of the shafts, and which engages external stationary elements. Stopping pins of this sort are shown in FIG. 1 of U.S. Pat. No. 5,936,324 (item number 32); and FIG. 2 of U.S. Pat. No. 5,424,632 (item number 18).
When a stopping pin is used, the axial length of the shaft must necessarily be extended to make room for it. A hole is drilled in the shaft where the stopping pin resides. Although the stopping pin does largely fill the hole, it does not completely fill the hole. Therefore, the combination of a longer shaft plus the hole drilled for the stopping pin weakens the shaft, and undesirably lowers torsional and bending-mode resonant frequencies.
When using this type of conventional actuator for optical scanning applications, the costs involved in forming, inserting, and retaining the coil present a genuine limit to how inexpensive an optical scanner can be made, and this limit has prevented certain consumer-grade laser scanning applications from flourishing. For this reason, there is clearly a need for an electromechanical rotary actuator that generally provides all of the benefits of this type of conventional actuator for optical scanning applications, while also having lower manufacturing costs.
With reference again to FIGS. 7 and 8, one known actuator illustrated in U.S. Pat. No. 4,319,823 is designed for camera shutter applications. In this actuator, the coil is rectangular and surrounds the magnet, and a shaft is attached to the shaft using an intermediate, U-shaped member. Unfortunately, because of the way only a single shaft is used and the way in which the shaft is attached to the magnet, this actuator could not be used for high-performance optical scanning applications, especially if those applications also required rotor position information.
The above referenced patent publications including: U.S. Pat. No. 4,090,112 for Electrically Damped Oscillation Motor (apparently the first “moving magnet” type of optical scanner); U.S. Pat. No. 5,313,127 for Moving Magnet Motor (a moving magnet type actuator); U.S. Pat. No. 5,424,632 for Moving Magnet Optical Scanner with Novel Rotor design to Montagu (a moving magnet scanner and rotor assembly having a stopping pin); U.S. Pat. No. 5,936,324 to Montagu for Moving Magnet Scanner (motor employing stopping pin item); U.S. Pat. No. 6,633,101 to Stokes for Moving magnet Torque Motor (an actuator: U.S. Pat. No. 7,365,464 to Brown for Composite Rotor and Output Shaft for Galvanometer Motor and Method of Manufacture Thereof (a rotor assembly method similar to Montagu); and U.S. Pat. No. 8,569,920 to Ramon et al. for Small Electric Motor (commonly used rotor assembly and method) are presented by way of examples and are herein incorporated by reference in their entirety.